1. Field of the Invention
The present invention relates to a DC level control method, a clamp circuit, and an imaging apparatus including the clamp circuit.
2. Description of the Related Art
When handling various electric signals including imaging signals, sometimes such handling is conducted by holding a DC level of an electric signal at a constant value. A circuit used for such purpose is called as a clamp circuit.
It is as well-known that a clamp circuit of an analog type is so configured that the clamp circuit samples an electric signal to be handled at a predetermined timing, stores a sampled level in a condenser (a storage element or a capacitor), compares the sampled level (an electric signal level at the time of sampling) with a pre-arranged reference level, and carries out a feedback operation so that a difference between the two levels will reach approximately zero.
In order to achieve a stability of the DC level in the electric signal, it is preferable to set a period of holding the sampling level to some degree of length. Further, to avoid any too sensitive reaction to noise during a period of clamp pulses, a condenser having a relatively large capacity is required as a sampling hold condenser in the analog type clamp circuit. Consequently, an external condenser is generally put to use, so that the problems remains in terms of the number of parts and mounting of parts.
For example, a clamp circuit for holding the DC level of an imaging signal outputted from a solid state imaging device to be a constant value is explained. In an imaging system using a solid state imaging device, a clamp circuit, by adjusting an OPB (Optical Black) level of an imaging signal with a reference level, prevents problems such as black floating or black sinking in the imaging signals, and realizes to secure a dynamic range of an analog circuit such as a CDS (Correlated Double Sampling) circuit.
Nevertheless, in a case where noise appears on this OPB level, there occurs a problem of noise generation in a picture image due to fluctuation of the clamp level. As a result, normally, it is so configured that the clamp circuit uses a condenser having large capacity as a device for holding the clamp level and concurrently acting as a circuit for performing the function of an LPF (Low Pass Filter), and a negative feedback is applied during the period of the clamp pulses to make the difference between the OPB level and a desired converging level as zero so that the clamp level clamped by the clamp pulse is maintained by the condenser.
However, the use of a condenser having too large capacity causes a slow response and deteriorates a picture image during a time period up to the time when the clamp level becomes stable at the time of initializing or of changing its gain, and this becomes noticeable in the picture image. Hence, such condenser is determined its capacity within a range permissible by considering a response rate. But this results, in the end, that the condenser having capacity large enough to achieve the LPF function cannot be utilized, and this causes a problem of horizontally extending noise on the screen. Furthermore, since a condenser having large capacity is unable to be mounted together with a solid state imaging device, and usually such a large condenser is treated as an external component part, so that improvement has been expected to reduce a mounting area as well as the number of parts.
On the other hand, in addition to the method of holding the analog value by the condenser to hold the clamp level, there is considered a clamp circuit of a digital type to hold the clamp level in a digital value. In this case, it is so configured that the clamp level is digitized by a multi-bit A/D converter and that a digital filter having bit redundancy or the like is used to perform digital filter processing, after that, a D/A converter is used for reconverting the digitized clamp level to the analog representation to carry out the feedback.
Nonetheless, in this case, the use of a multi-bit A/D converter and a digital filter circuit causes to enlarge the size of the circuitry. Further, there will be an increase in the digital noise level and the frequency of the noise will be relatively high, thus creating a problem of digital noise affecting the clamp circuit system.
For example, an OPB clamp of an imaging apparatus is discussed. In the case of a the digital type clamp circuit for digitally holding the OPB clamp level, it is so configured that the clamp level is digitized by a multi-bit A/D converter for signal processing to perform digital filter processing, after that digitized clamp level is again subjected to the analog representation by the D/A converter to carry out the feedback. In this case, the multi-bid A/D converter for the signal processing naturally requires a high sampling frequency, so that a frequency of the digital noise generated becomes higher. Further, because of the larger scale of the multi-bit A/D converter and the digital filter circuit, it becomes necessary to examine the extent of mounting them on the same substrate on which the solid state imaging device is mounted.
When mounting all of the A/D converter, the digital filter circuit, and the D/A converter altogether on the same substrate on which the solid state imaging device is mounted, any connection to a signal processing LSI will be made via a digital bus. However, for application such as a small module, a multi-bit digital bus requires some size of substrate area therefore, thus this makes it disadvantageous in term of substrate size. Furthermore, there is such a large amount of noise generated as to make it difficult to be employed. This is also applicable to a case of mounting only an A/D converter on the same substrate on which the solid state imaging device is mounted.
On the other hand, when all of an A/D converter, a digital filter circuit, and a D/A converter are mounted together with a signal processing LSI of the latter stage, there is a fear that digital noise is induced on an analog signal line subsequent to the D/A converter and mixed in a solid state imaging device through a feedback bus. In an digital imaging system where an A/D converter, a D/A converter, a digital filter, and a solid state imaging device are mounted together on the same substrate, it is impossible for a signal processing LSI and a solid state imaging device to be considered separately as a system. This system cannot be employed if there is a possibility of using a general-purpose product for a signal processing LSI.
In this manner as described above, a conventional clamp circuit of the digital type is designed as a combination of a clamp circuit with a multi-bit A/D converter for signal processing, or a D/A converter, and therefore, there exists a problem that a complete form of the clamp circuit is not realized yet in terms of noise and circuit arrangement.